RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Schedule of Significant Unobservable Inputs Used in the Fair Value Measurement of Level 3 Derivative Instruments (Details) $ in Millions	Jun. 30, 2025 CAD ($) $ / MMBTU $ / bbl $ / MWh	Dec. 31, 2024 CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ (926)	$ (2,250)
Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	(926)	(2,250)
Level 3
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	17
Level 3 | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ 17	$ (52)
Commodity Options | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	3
Commodity Options | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	74
Commodity Options | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	52
Natural gas | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	2.31
Natural gas | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	8.99
Natural gas | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	5.22
Natural gas | Commodity Contracts - Financial | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ 8
Natural gas | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	0.35
Natural gas | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	13.79
Natural gas | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	4.42
Natural gas | Commodity Contracts - Physical | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ (45)
Natural gas | Commodity Options | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	3.62
Natural gas | Commodity Options | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	11.03
Natural gas | Commodity Options | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent)	7.22
Natural gas | Commodity Options | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ 69
Crude | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	63.63
Crude | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	87.11
Crude | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	83.24
Crude | Commodity Contracts - Financial | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ 5
Crude | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	65.05
Crude | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	112.22
Crude | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / bbl	85.79
Crude | Commodity Contracts - Physical | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ (6)
Power | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	31.69
Power | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	185.09
Power | Commodity Contracts - Financial | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	69.54
Power | Commodity Contracts - Financial | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ (7)
Power | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	28.7
Power | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	139.72
Power | Commodity Contracts - Physical | Level 3 | Market Approach Valuation Technique | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Price volatility (in $/MMBTU, $/barrel, $ / MW/H, percent) | $ / MWh	70.66
Power | Commodity Contracts - Physical | Level 3 | Fair Value | Market Approach Valuation Technique
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value | $	$ (7)